Vascular graft prostheses allowing endovascular placement have come into use as an effective, minimally invasive method of repairing damaged or diseased vessels, especially major blood vessels such as the aorta. These prostheses are comprised of one or more sleeves of fabric-like graft material, such as polyester (e.g., DACRON(copyright), a trademark of DuPont Nemours and Co.), PTFE or collagen and metallic stents which are secured to the vessel wall to prevent-migration of the prosthesis, maintaining an open lumen therethrough, and to serve to seal the respective ends of the sleeve to the vessel wall to prevent leakage of blood around the sleeve ends to the outside of the sleeve. Proper sealing is especially critical when the prosthesis is used to bridge a segment of blood vessel that has been seriously compromised and can no longer prevent extravasation, such as in the case of an abdominal aortic aneurysm (AAA), a leading indication for prosthesis placement. The stents frequently include one or more barbs or projections that help anchor the prosthesis at the deployment site and prevent migration or trauma to the aortic neck.
The stents used to anchor the prosthesis, maintain an open lumen throughout, and seal the sleeve end, are preferably of the self-expanding stent type. The Z-Stent(trademark) (Cook Incorporated, Bloomington, Ind.) and other closely related zig-zag stents of the same basic pattern are used in a number of AAA endovascular grafts due to their excellent expansion ratio and ability to compress into a relatively small introducer catheter, such as 18-20 Fr (6.0-6.7 mm) for deployment through a small cut down, or percutaneous puncture to, an access vessel. These zig-zag stents have struts connected by bends. The zig-zag stents are sutured or secured along the sleeve and/or at the ends of the sleeve. In some devices, a stent is secured to the proximal end of the sleeve and the proximal end of the terminal stent is placed renally such that the top edge of the sleeve lies just below the renal arteries. Therefore, the terminal stent can be securely anchored near the renal arteries and being open, does not compromise blood flow to the renal arteries.
For deployment, the prosthesis is compressed into a deployment system. In one embodiment of a deployment system, the terminal stent is compressed and loaded in a tubular component of the deployment system. In the fully compressed state, the struts of the zig-zag stent are generally parallel, however during loading into the tubular structure, the bends do not assume a regular or even arrangement inside the cap. As a result, the compressed bends can become disoriented and entangled such that when the stent is deployed, the bends cannot fully expand and properly seal the vessel. This problem is greatly compounded if the terminal stent has barbs on some struts such that the barbs can snag the other struts, leading to an unacceptably high rate of deployment failure. In fact, this irregular orientation of bends during compression would be inherent in virtually any zig-zag stent or serpentine stent made of bent wires due to the properties of the wire, manufacturing techniques, variable degrees of stress held in the individual bends, etc., that would not allow for a predictable compression to a desired target orientation by standard means. The recent addition of barbs to terminal stents of prostheses, such as for the AAA repair, has especially brought about an appreciation of this problem and the search for a solution.
The foregoing problems are solved and a technical advance is achieved in an illustrative stent having the terminal bends angled with respect to each other in a loaded or compressed configuration such that the respective bends, including barbs, do not become entangled with one another during expansion of the stent. A further clinical advantage of this configuration is that the stent can be further compressed than would be otherwise possible with a random configuration of bends such that the stent can be introduced via a smaller diameter delivery system.
In one aspect of the invention, the individual apices of the bends are plastically deformed into the angled arrangement by twisting the terminal portions of the bends from their original orientation, in which all struts in cross-section generally lie end-to-end in a circular configuration, to an orientation where the angled apices or fillets overlap by a consistent amount (i.e, a fan blade-like arrangement) to provide increased separation between struts of adjacent bends. Another advantage of this configuration is that the struts can be brought in closer proximity to the center, thereby allowing reduction of the size of the delivery system. Bending of the apices can occur in a jig wherein pins and clamps secure adjacent bends, while an opposite bend is laterally twisted with an articulating pin and clamp to produce the final orientation of bends.
In a second aspect of the invention, the terminal apices of the stent are interconnected by a suture, thread, or other tying means and drawn together for loading into the introducer system. When the suture is threaded through each fillet in an identical manner (e.g., outside to inside), it forces the respective fillets to twist in the same direction as they are drawn together. While keeping the ends drawn tight, the stent is loaded into a tubular component of the delivery system. Preferably, an eyelet or viewing portal in the side of the introducer is used to ascertain that all of the apices are visible and properly aligned. The suture is removed after the stent is loaded. This method of loading produces the same orientation of the terminal apices as in the pre-twisted configuration without having to plastically deform the bends. Other methods for either permanently or temporarily orienting the bends into an angled arrangement are contemplated to achieve a similar goal.